This invention relates to a method and apparatus for controlling a current type inverter, more particularly a current type inverter provided with a forced commutating circuit and utilized to drive an AC motor or the like load.
FIG. 1 shows a so-called series diode type inverter of the well known type. The inverter circuit comprises a DC source 1, a series reactor 2, main thyristors 21-26, diodes 31-36, and commutating condensers 41-46. The inverter bridge circuit comprises three parallel connected branch circuits each including two main thyristors and two diodes which are connected in series as shown and the commutating capacitors are connected between adjacent branch circuit. A load, shown as a three phase motor 4 is connected to the junctions of a pair of diodes in each branch circuit. There are also provided a diode bridge 6, a condenser 7, and a resistor 8, the AC terminals of the diode bridge 6 being connected in parallel with the motor 4.
The DC current from the DC source 1 is converted into a three phase AC by the operation of the inverter bridge and supplied to phases U, V and W of the motor 4. The operation of the inverter bridge at the time of commutation is as follows: Let us consider an interval in which current is flowing through main thyristor 21, diode 31, phases U and W of the load 4, diode 36 and main thyristor 26, and in which the current is to be communicated from phase U to phase V.
Before commutation, the condenser 41 is charged to have a polarity as shown in FIG. 1. When thyristor 22 is ignited under these conditions, the main thyristor 21 will be turned off by the voltage across the commutating condenser 41 so that the current from DC reactor 2 flows to phase U through main thyristor 22, condenser 41 and diode 31 thereby charging the condenser to a polarity opposite to that shown in FIG. 1. When the terminal voltage of the condenser 41 exceeds the voltage between U and V phases, diode 32 begins to conduct whereby the condenser 41 constitutes an oscillation circuit together with the inductance of the motor thereby decreasing the U phase current while increasing the V phase current. When these currents become equal to the direct current I.sub.DC, the U phase current becomes zero, thus completing commutation to phase V.
With this construction, however, the energy stored in the inductance of motor 4 has a tendency to overcharge the commutating condenser 41 so that its terminal voltage becomes excessive. To prevent this, the diode bridge 6 is connected to the load terminals and the capacity of the condenser 7 is made large so as to absorb the excessive voltage of the condenser 41, the charge of the condenser 7 being discharged through resistor 8, or regenerated back to the source 1 by another inverter, not shown.
However, absorption of a portion of the commutating energy by the condenser 7 and discharge of its charge through a resistor means loss of energy, and even where the charge in the condenser 7 is regenerated by an independent inverter, the cost of the inverter circuit increases.
Accordingly it has been proposed an improved current type inverter in which a portion of the commutation energy is stored in a condenser and the charge therein is utilized again for commutation.
FIG. 2 shows such current type inverter, which, in addition to the main inverter bridge 3, comprises an auxiliary thyristor bridge circuit 5 acting as a switching circuit and including first auxiliary thyristors 51-56, a second auxiliary thyristors 11 and 12, thyristors 13 and 14 acting as choppers, a condenser 17 connected between these thyristors 13 and 14 and diodes 15 and 16 connected as shown.
With the circuit shown in FIG. 2, DC current from the source 1 is converted into three phase alternating current by the main inverter bridge circuit 3 and applied to the load motor 4. The AC terminals of the main inverter bridge circuit 3 is connected to corresponding AC terminals of the auxiliary thyristor bridge circuit 5, whereas the second auxiliary thyristors 11 and 12 are arranged to interconnect DC terminals of the main inverter bridge circuit 3 and the auxiliary thyristor bridge circuit 5.
A series circuit comprising the choppers 13 and 14 and the capacitor 17 (or an auxiliary DC source) is connected across the DC terminals of the auxiliary thyristor bridge circuit 5. The purpose of the diodes 15 and 16 is to connect, with a reverse polarity, the condenser 17 across the DC terminals of the auxiliary thyristor bridge 5 when the chopper 14 is turned off.
In many cases the DC source 1 is constituted by a phase controllable three phase thyristor rectifier and the choppers 13 and 14 are constituted by gate turn off thyristors (GTO), thyristor choppers provided with commutating devices or transistors.
A prior art method of controlling the inverter device shown in FIG. 2 will now be described with reference to the waveforms shown in FIG. 3 and diagrams shown in FIGS. 4a-4c. Curve A shown in FIG. 3 shows U phase current I.sub.u of the motor 4, curves B and C the V phase and W phase currents respectively, curve D the ON and OFF states of the second auxiliary thyristor 11, curve E the ON and OFF states of the choppers 13 and 14, curve F the ON and OFF states of the second auxiliary thyristors 12, curve G the voltage V.sub.c of condenser 17, curve H the ON.OFF state of the main thyristor, curve I the ON.OFF state of the first auxiliary thyristor, curve J the ON.OFF state of the main thyristor, curve K the ON.OFF state of the main thyristor, curve L the ON.OFF state of the auxiliary thyristor 53, and curve M the ON.OFF state of the main thyristor.
FIGS. 4a, 4b and 4c show current flow states under various control conditions. At time t.sub.0, the main thyristors 21 and 26 are ON and the current flows through motor 4 in a direction shown by an arrow in FIG 4a. Under these conditions, when the second auxiliary thyristor 11 and choppers 13 and 14, and the first auxiliary thyristor 55 are turned ON at time t.sub.1, a state as shown in FIG. 4b is established so that condenser 17 discharges to decrease V.sub.c. At time t.sub.2, the U phase current I.sub.u becomes zero whereas the V phase current I.sub.v becomes equal to the direct current I.sub.d. During an interval between t.sub.2 and t.sub.3 the voltage of condenser 17 is applied in the reverse direction across the main thyristor 21 so that this thyristor is turned OFF. Then at time t.sub.3 choppers 13 and 14 are turned OFF, the condenser 17 is charged by the V phase current I.sub.v to increase its voltage V.sub.c. At time t.sub.4 the main thyristor 22 is turned ON. Then the divided voltage of condenser 17 is applied across the second and first thyristors 11 and 55 in the reverse direction so as to turn OFF them. This state is shown in FIG. 4c.
The interval TD between t.sub.1 and t.sub.3 during which choppers 13 and 14 are ON should be made sufficiently longer than the commutation interval between A.sub.1 and A.sub.4 by taking into consideration variation in the load current as well as the variation in the backelectromotive force of the load. Because if the commutation completion time t.sub.2 becomes later than time t.sub.3 commutation failure would be resulted.
The timing of commutation from the first auxiliary thyristor 55 to the main thyristor 22 is controlled by detecting the fact that the voltage of condenser 17 has returned to the original value by which time t.sub.4 is determined. This is done for the purpose of supplementing the loss of the charge of condenser 17 at the time of commutation during the interval t.sub.3 and t.sub.4. Since at time t.sub.4 the condenser voltage V.sub.c has restored the value of t.sub.1, such supplement assures the next commutation. In the same manner, during an interval between t.sub.5 and t.sub.8 current is commutated from main thyristor 26 to main thyristor 24.
As can be readily understood from FIG. 3, the interval between t.sub.2 and t.sub.3 is relatively longer than the commutation interval t.sub.1 to t.sub.2 and moreover it takes a relatively long time to recover the condenser voltage V.sub.c so that in order to operate with a commercial frequency it is necessary to select the capacitor voltage to be about twice of the load or motor voltage. This requires to make high the withstand voltage of the main circuit elements, which is uneconomical. More particularly, as the commutation interval t.sub.1 -t.sub.2 is proportional to the load current and the load inductance and inversely proportional to the difference between the condenser voltage V.sub.c and the back electromotive force of the load so that it is necessary to increase the condenser voltage V.sub.c. Increase in the condenser voltage causes increase of the rate of change in the load current at the time of commutation thus not only increasing noise of the motor but also increasing the commutation surge voltage at the load terminals. This requires increase in the breakdown voltage of the motor winding. Accordingly, it has been desired to provide an improved method of controlling an inverter device capable of increasing the operating frequency thereof by decreasing the time of completing the commutation sequence without increasing the condenser voltage.